The present invention relates to a safety apparatus for an elevator in which a hoist motor is driven and controlled by a drive controller employing the so-called inverter.
In elevators, there has heretofore been an apparatus wherein an induction motor is used as a hoist motor for raising and lowering a cage and is driven and controlled by a drive controller employing an inverter.
Such apparatus for driving and controlling the elevator is as shown in FIG. 1 by way of example.
This drive and control apparatus is such that a three-phase A.C. output from a three-phase A.C. power source 1 is applied through a switch 2 to a converter 3, by which the three-phase A.C. output is converted into a D.C. output.
The D.C. output of the converter 3 is smoothed by a capacitor 4 and then applied to a transistor inverter 5. The D.C. output is inverted by the inverter 5 into a three-phase A.C. output, which is applied to a hoist motor 6 which is an induction motor so as to drive the motor.
Thus, a sheave 7 which is coupled to the rotary shaft of the hoist motor 6 is turned. A traction rope 9, which is wound round the sheave 7 and which has a balance weight 8 attached to one end thereof, has a cage 10 coupled to the other end thereof, so that the cage 10 ascends or descends with the turning of the sheave 7.
The apparatus further comprises: a regulator 15 which is an inverter controller which is supplied with a command velocity signal P.sub.A from a pattern generator 12 for generating a velocity pattern for the cage 10, a detected velocity signal P.sub.B from a tachometer generator 13 for detecting the rotational velocity of the hoist motor 6 and the running velocity of the cage 10, and a detected current signal I.sub.A which is a feedback signal from a current detector 14 for detecting the three-phase output current of the inverter 5.
Thus, the regulator 15 base-drives the respective transistors of the inverter 5 alternately on the basis of these input signals P.sub.A, P.sub.B and I.sub.A, to control the D.C. - A.C. change and to control the rotational velocity of the hoist motor 6 and, accordingly, the running velocity of the cage 10.
As shown by way of example in FIG. 2, the regulator 15 is composed of a phase compensation circuit 16 which compares and operates on the command velocity signal P.sub.A from the pattern generator 12 and the detected velocity signal P.sub.B from the tachometer generator 13, a PWM comparator circuit 17 which compares a command current signal I.sub.B, which is the output of the phase compensation circuit 16, and the detected current signal I.sub.A from the current detector 14 and which provides a pulse width modulated pulse (PWM), and a base drive circuit 18 which is constructed of transistors etc. to be controlled "on" and "off" by the output pulses from the PWM comparator circuit 17 and which delivers a base drive signal P.sub.C to the inverter 5.
In such drive and control apparatus, when the current detector 14 for detecting the output current of the inverter 5 has malfunctioned, it fails to proyide the detected current signal I.sub.A. Therefore, the regulator 15 continues to deliver the base drive signal P.sub.C for driving the respective transistors of the inverter 5 into the "on" state, and the output of the inverter 5 becomes saturated, so that the respective transistors of the inverter 5 may be damaged due to an overcurrent.
This drive and control apparatus is therefore further provided with safety device, namely, a current detector 19 which is disposed on a feeder line to the inverter 5 and which detects current flowing through the inverter 5, and an overcurrent detector 20 which supplies the base drive circuit 18 of the regulator 15 with an operation stopping signal P.sub.D, namely, a signal for disabling the respective transistors of the base drive circuit 18 from turning "on", when the detected current value of the current detector 19 has exceeded preset value, that is, when an overcurrent flows through the inverter 5.
In this way, when an overcurrent flows through the inverter 5, the base drive signal P.sub.C is not delivered from the base drive circuit 18 of the regulator 15 to the respective transistors of the inverter 5, and the inverter 5 fails to operate, so that damage of the transitors of the inverter 5 can be prevented.
In such a case, however, the current detector for detecting the current flowing through the inverter is required separately from the current detector for detecting the output current of the inverter, which results in the drawbacks of a complicated arrangement and a high cost.